Hydrocolloid gums of the type contemplated by this invention have traditionally been recognized for their ability to bind water, provide increased viscosity, suspend solids, protect emulsions, and retard crystalization, including ice crystal growth in frozen foods. As used herein, the term "hydrocolloid gum" is meant to include such colloids as pectin, algin, carrageenin, xanthan, cellulose ether, microcrystalline cellulose, karaya and agar. Some of the starch/hydrocolloid blends envisioned in this invention as simple mixtures of dry powders have been used in the past to modify the properties obtainable with any of the components used alone. For example, U.S. Pat. No. 4,064,282 describes a starch sponge which is given added stability by the addition of hydrocolloid gums, U.S. Pat. No. 4,415,599 describes a starch/guar gum blend useful as a gravy or sauce mix, and U.S. Pat. No. 3,955,009 proposes a pre-gelatinized starch, acid and hydrocolloid gum blend as a dry dessert mix composition.
It can readily be demonstrated that such mechanical (i.e. un-processed) starch/gum blends exhibit properties that are certainly not synergistic but, rather, additive at best. The present invention discloses a process whereby the desirable properties of a blend of starch and guar, for instance, can be greatly enhanced over those shown by mere mechanical mixtures.
Whereas unprocessed starch will lose a great deal of its water of hydration upon freezing, the blends processed as described in this invention are well-suited for stabilizing frozen desserts, such as ice cream, ice milk and sherbet. Furthermore when guar gum is used, either alone or together with unprocessed starch, its stabilizing properties are not as long-lasting as that produced by the more expensive hydrocolloid gums, such as locust or cellulose gum and, at high levels, guar imparts a "stickiness" which is undesirable. As this invention demonstrates, a processed blend of starch and guar gum overcomes both of these shortcomings, i.e., it gives longer-lasting heat-shock protection together with a "shorter" body.
Among the many patented processes for chemically modifying hydrocolloids such as starch, guar and locust gum to alter their physical properties, are the following: to cross-link starch, U.S. Pat. No. 3,878,196 uses POCl.sub.3 while U.S. Pat. No. 3,705,046 uses formaldehyde; starch ethers or esters have been prepared according to U.S. Pat. No. 3,728,332 using N-stearylguanidine, and according to U.S. Pat. No. 3,706,730 using benzyl chloride; carboxyalkyl ethers of galactomannan gums have been prepared using sodium chloroacetate according to U.S. Pat. Nos. 3,712,883, and 3,740,388.
Chemical modification may involve cross-linking of the hexose chains, esterification or etherification of some hydroxyl groups, or oxidation of some of the primary hydroxyls to carboxyls but in every case covalent bonds are added or altered to accomplish the desired result. For this reason, in the preliminary investigations which led to the present invention, it was assumed that covalent linkages would be required between the two types of hydrocolloids, such as the starch and gum, if a significant degree of synergism was to be achieved. Therefore, it came as a complete surprise to discover processing conditions under which a negligible amount of new covalent links are formed and yet the starch and gum are bonded together in such a way as to yield new and valuable properties.
Taking a 50/50 mechanical blend of fine-mesh guar and starch as an example, it is believed that upon hydration with an excess of water up to the boiling point and up to the maximum practical usage level (about 3% in foods), each particle swells independently. None of the end-product processing conditions are believed to favor the formation of colloid-to-colloid bonds over colloid-to-water hydrogen bonds. This is believed true for food products processed at ambient pressure or retorted, just as it is believed true for industrial products, such as textile dyes, paper sizes, and oil drilling muds. Even the high shear homogenization used in ice cream mixes does not serve to combine the components of this mixture, but is believed to merely degrade the starch viscosity with the latter being especially true in low fat formulations.